The present invention relates to a motor drive unit utilizing an elongated threaded drive rod and limit sensors for controlling the position of a pusher block with respect to the drive rod and particularly to a mounting and cover assembly for the limit position sensors and drive assembly.
In work places, it is frequently desirable to provide a working surface with an adjustable height. In order to either retrofit existing work benches or tables and/or provide originally adjustable table heights, one system has employed a cylinder lift system in which fluid cylinders are attached to each of the work surface legs. A fluid pump is coupled by tubing to each of the cylinders and controlled either by a manual control or by a motor-driven assembly. Such systems are commercially available from Suspa Incorporated, the Assignee of the present invention, under the brand MOVOTEC(copyright) lift systems. Such systems provide readily retrofittable work surfaces which can lift loads in the range of from about 0 to about 1500 pounds depending upon the size of the system. They provide an adjustment range of from about 0 mm to about 400 mm. In place of a manually controlled pump, motor-driven units have become increasingly popular in which an electrically driven motor can be actuated to rotate a threaded rod on which a threaded pusher block is mounted. The pusher block, in turn, is coupled to the push rods of cylinders for applying pressure to fluid lines coupled to the individual leg adjusting cylinders. This system provides the advantage of easier and faster operation but requires safety features to prevent motor burnout or preventing stripping of threads of the pusher block relative to the threaded rod at the end of travels of the pusher block.
In the past, limit switches have been mounted adjacent an open slot associated with an extrusion holding the pusher block cylinders and threaded rod such that as the pusher block reaches the opposite ends of its travel limits, limit switches are actuated to disconnect power to the motor until a command is sent to the control unit for operation in an opposite direction. The motor-driven unit typically is mounted to the under surface of the table with the exposed slot allowing the mounting of limit switches in a position to be engaged by a projection on the pusher block guided within the extrusion at its travel limits. Although such systems provide the desired operational features, the threaded power screw typically is greased to provide smooth operation over a long period of time and can tend to become, depending upon the environment in which the system operates, contaminated due to the open mounting slot for the travel limit switches. Also, the open slot allows the noise from the operation of the power screw to be transmitted to the surrounding environment, making the operation of the table somewhat noisy, particularly in relatively quiet working environments.
Thus, there exists a need for a system by which a motor-driven table lift system can employ a sensor system which provides control signals for the travel limits of a pusher block associated with the drive system and reduce the noise while protecting not only the greased power screw but the sensors and control wires while positioning the sensors along the slot at desired locations.
The system of the present invention satisfies this need by providing a position sensor holder which comprises an elongated track which can be positioned over the slot in the housing of a motor-driven lift assembly and which includes sockets for receiving and holding position sensors which respond to a permanent magnet mounted to the pusher block for providing signals to a control system indicating the travel limits of the pusher block within the lift system housing.
In the preferred embodiment of the invention, the position sensor holder comprises an elongated, extruded polymeric member having a mounting surface on one side for attachment to the lift system housing and an elongated socket for receiving and holding magnetically responsive sensors, such as a reed switch or Hall-effect device, positioned on an opposite side in proximity with the slot and a magnet positioned in the pusher block such that the proximity of the magnet to the sensor actuates the sensor for providing a control signal at the travel limits of the pusher block. In a preferred embodiment also, the socket comprises an elongated, continuous track which allows adjustment of the sensor for precisely locating the sensors for desired travel limits. In a preferred embodiment also, the mounting socket comprises a pair of upwardly extending resilient legs for snap-receiving the sensors and including an undercut for receiving a cover element which covers the sensors and wires extending along the elongated socket coupling the sensors to the control system.
Thus, with the system of the present invention, a position sensor holder locates and positions travel limit sensors on the lift assembly housing, covers the power screw contained within the housing against environmental contamination, reduces noise which otherwise would escape from the open slot in the housing, holds and protects the sensors and associated wires to the housing. A snap-fit, resilient polymeric cover fits over the position sensor holder in the preferred embodiment to provide further protection for the sensor mounting system once assembled.
These and other features, objects and advantages of the present invention will become apparent to those skilled in the art upon reading the following description thereof together with reference to the accompanying drawings.